The invention concerns a so-called skull pot for melting or refining glass or glass ceramics.
Such pots are surrounded by a pot wall, which is normally cylindrical. It consists of a ring of vertical metal pipes, where a gap remains between adjacent pipes. The base of the pot may also be constructed of metal pipes, but it may also consist of fireproof material.
The heat source involves an induction coil, which surrounds the pot wall and through which high-frequency energy can be coupled into the contents of the pot.
An example of such a skull pot was made public by EP 0 528 025 B1.
The skull pot operates as follows: The pot is filled by batch or cutlet or a mixture thereof. The glass or the melt needs to be preheated initially to achieve a certain minimum conductivity. This preheating is often accomplished by blowpipe heating. Once the coupling temperature is reached, the additional input of energy involves irradiation with high frequency energy. It is also possible to heat the melt by burners, which heat the melt from above, in addition to the heating via high frequency energy. The additional heating of the melt surface may also be done by electrical heating elements.
A margin layer of solidified melt forms on the cooled pot wall, which consists of metal pipes, during operation. This protects the pot wall from corrosion from aggressive or hot melt. This cold margin layer may either be glassy or crystalline, depending on the glass melt.
Likewise, the base layer is also cold, because the base is cooled just as the wall is cooled. A glassy or crystalline cold base layer is formed there as well. This is not advantageous for run-off of the melt via a base drain. To be specific, either a hole would have to be punched in the solidified base layer or it would have to be dissolved by means of additional heat sources. Here the crystalline layer acts as a nucleating agent for the melt rushing past, which is undesirable.
The high frequency energy can be used only for the heating of the skull pot interior. However, it is not possible to use it for targeted heating of the cooled base layer. If the induction heating system were to heat the base layers, these layers would lose heat by the action of the base coolant. This would reduce the energy efficiency, compared to the un-cooled hot center zone of the melt.
It could be argued that the high frequency input should be raised sufficiently overall, such that the temperature of the base area exceeds the upper divitrification temperature. Admittedly, this would solve the problem of achieving run-off. However, this would also overheat the melt in the center of the skull pot. This could lead to a shift of the synthesis by the selective evaporation of certain glass components, which would induce fluctuations in the refractive characteristics and streaks.
Thus, it needs to be noted that the relatively low temperatures along the bases of conventional skull pots are detrimental.
It may also be just as detrimental if the surface area of the melt in refining vessels is relatively cold. After all, the cold surface layers of the melt would restrict bubbles from rising and bursting.
But high surface temperatures of the melt are desirable not only during refining, but also during melting, because they favor quick melting.
The invention focuses on finding measures to modify the skull pot in such a way that the temperature of the melt in various segments on various geodesic levels can be controlled, given a constant energy input during the production process. Thus, it should be possible, for instance, to induce an especially significant increase of the temperature of the melt in the lower or in the center or in the surface area of the pot. As an alternative, it should be equally possible to generate an improved homogeneity of the temperature across all levels of the entire pot.
This task is solved by the characteristics of Claim 1.
The inventors have realized the following:
The high frequency radiation is also absorbed by the metallic cooling pipes, but to a lesser degree. This induces voltage differentials among the cooling pipes, which are isolated electrically from each other. This generates arcing between individual cooling pipes.
This is particularly noticeable if the solidified cold layer of glass along the wall is thinned significantly due to high melting temperatures.
In that instance, the full induction voltage acts on these layers, because the conductivity of the hot melt and the conductivity of the cooling pipes is significantly higher than the conductivity of the cooled wall layer of glass. The reduced wall layer thickness leads to high field intensities and thus to the generation of the aforementioned arcing. The arcing damages the metal cooling pipes and will destroy the pot, in the long run.
The invention, specifically the inclusion of a short-circuit ring, will prevent the arcing. This will increase the useful life of the pot.
It will be necessary in very large skull pots with long cooling pipes to include a short-circuit ring at the top and one at the bottom of the pipes. A single short-circuit will not suffice to defuse the high frequency induction voltages.
This applies particularly to melts, where there is no stable, crystalline and electrically isolating margin layer between melt and skull wall. The use of a double short-circuit allows for the precise fine-tuning of the field distribution, and thus the temperature distribution, by varying the distance at which the short occurs from the center of the melt.
For example, the short may be induced near the base of the pot on a variable basis by not connecting the cooling pipes of the skull pot directly, but rather extending the cooling pipes downward by threaded pipes, where an adjustable short-circuit ring is attached to the threaded pipes. The optimal position for the short can then be selected, depending on the melting phase (melting, refining or casting). A comparable, height-adjustable modification of the short can be positioned on the upper pot rim as well, of course.
For very tall pots, a second short-circuit may be required under certain circumstances, but a second short-circuit may also make it feasible to pull one of the two short-circuits a significant distance away from the pot. This applies particularly to the melting phase of the process, when relative low melting temperatures need to be used and when the cold glass margin layer forms normally even in critical glass systems.
However, the inventors have also realized the following:
When the metal cooling pipes are short-circuited together, the location of the short tends to push the high frequency field either upwards or downwards. If the short is at the base of the pot, the displacement is towards the top. If the short is in the upper segment, the displacement is towards the base. When the field is pushed towards the top, the temperature of the surface layer of the melt is increased. When the field is pushed towards the base, the temperature of the base layer is increased. Thus, it is feasible to increase the temperature of the melt in a specific layer, or conversely to achieve a homogeneous distribution of temperature, by adjusting the location of the short.
The short would normally be adjusted towards the base during refining. In this case, the high frequency field is pushed towards the top. The surface layer of the melt is heated to particularly high temperatures. The gas bubbles contained in the melt willingly ascend, easily permeate the surface layer and burst.
During melting, it may also be advantageous to move the short towards the base. This leads to particularly effective heating of the developing surface layer, which facilitates melting.
The run-off should be free of crystals during casting. The melt should have a viscosity of roughly 104 dPas in the run-off area, where the melt in higher pot layers does not need to be heated to refining temperatures, which would lead to thermal reboiling. The short should be moved to the upper area of the pot in order to keep the run-off free of crystals during casting. In this case, the high-energy field is pushed downwards, so that the run-off level is heated particularly vigorously.
It is most advantageous, if the short is caused by a metallic ring, which connects the metal cooling pipes by an electrically conductive medium. The short, regardless of its physical nature, must be located somewhere along the length of the cooling pipes, i.e. between the base of the skull pot and the upper terminus of the cooling pipes. If the short is located outside of this range, it is not effective for high-frequency voltage, because the inductive resistance of such a xe2x80x9cwire loopxe2x80x9d is too large. U.S. Pat. No. 4,049,384 describes a skull pot that consists of two modules. Each module contains a string of cooling pipes, which are arrayed in a semi-circle and which are firmly attached to a metal base plate that also has a semi-circular shape. The two base plates are joined with an electrically isolating spacer, to avoid a short between the modules, as the patent descriptions points out at length. This is consistent with the historic presumption that a short must be avoided, contrary to the basic concept of the present invention.
The fear may arise that a short would consume more high-frequency energy in the skull pot itself or that the high-frequency field would not penetrate the interior of the pot. Such fears have not been shown to be realistic. The additional consumption of high-frequency energy is minimal. Thus, the invention has major advantages, but just about no disadvantages.
The increase in mechanical stability of the pot, given the construction according to the invention with a short-circuit ring, is an additional advantage.